In some applications, it can be desirable to form a uniform coating over a non-circular substrate. For instance, it can be desirable to coat a uniform layer of photoresist on a square or rectangular shaped radiation patterning tool, such as, for example, a reticle or mask.
One method of coating a radiation patterning tool is to spin the radiation patterning tool while flowing a liquid thereover. The liquid is commonly applied to the center of the radiation patterning tool and concentrically spreads outward in radial directions due to the action of centrifugal forces during spinning. A difficulty in utilizing the above-described spin coating method for applying liquid over non-circular shaped substrates (such as rectangular substrates) is that the non-circular periphery of the substrates can cause turbulent airflow during spinning that affect the uniform flow of the liquid. The turbulent airflow exerts undesirable aerodynamic forces to the liquid, causing non-uniform flow of the liquid on the substrate. Such can cause the thickness of the liquid to vary across the substrate.
In an exemplary process wherein a liquid photoresist is spun over a non-circular radiation patterning tool, the uneven distribution of the liquid can diminish the accuracy with which the photoresist is subsequently patterned. Such diminished accuracy of photoresist patterning can in turn lead to diminished accuracy in subsequent patterning of the radiation patterning tool. It would, therefore, be desirable to develop improved methods for spin coating non-circular substrates.